Oxygen depletion under glass: Behavioural responses of benthic macrofauna to induced anoxia in the Northern Adriatic

The Northern Adriatic Sea experiences seasonal low dissolved oxygen (DO) events and mortality of benthic organisms. The onset and extent of such disturbances, however, are difficult to forecast and study in the field. We address this problem by deploying a device that artificially induces and documents small-scale anoxia on soft-bottom in 24 m depth. Behaviour changes and mortality sequences of the soft-bottom macroepi- and infauna over a 5.5-day deployment were documented. An intervening oxygen peak created two phases of declining oxygen values. All initiated behaviours were interrupted during the reoxygenation and predictably re-appeared in the second phase, demonstrating the direct relationship between oxygen levels and behaviour. Beginning hypoxia (≤ 2.0 ml l- 1 DO) elicited escape patterns such as increased horizontal and vertical locomotion. Moderate hypoxia (≤ 1.0 ml l- 1 DO) triggered species-specific sublethal effects such as arm-tipping in ophiuroids or extension from the sediment in sea anemones. At severe hypoxia (≤ 0.5 ml l- 1 DO) infaunal organisms began to emerge and first mortalities occurred. The crustacean Macropodia sp., the sea urchin Psammechinus microtuberculatus and the brittle star Ophiothrix quinquemaculata were among the first to die; the sea anemone Cereus pedunculatus and the gastropod Hexaplex trunculus survived. Hydrogen sulphide (H2S) increased until the end of the deployment to 163 µM; temperature in the chamber remained nearly constant at 18.5 °C.This design provides insight into species and community-level processes and is an important step in identifying potential indicator species for low DO events in the Northern Adriatic and elsewhere.